Heat developable photographic materials and processes have been well known in the art for many years. Photosensitive, heat-developable, dry silver sheet materials, as described for example in U.S. Pat. Nos. 3,457,075 and 3,839,049, contain a photosensitive silver halide catalyst-forming means in catalytic proximity to a heat sensitive combination of a light stable organic silver compound and a reducing agent therefor. When struck by light, the silver halide catalyst-forming means produces silver nuclei which serve to catalyze the reduction of the organic silver compound, e.g., silver behenate, by the reducing agent at elevated temperatures.
A variety of processes for obtaining colour images have been proposed.
U.S. Pat. No. 4,021,240 discloses the use of sulphonamidophenol reducing agents and four equivalent photographic colour couplers in photothermographic emulsions to produce dye images.
U.S. Pat. No. 4,022,617 discloses the use of leuco dyes in photothermographic emulsions. The leuco dyes are oxidised to form a colour image during the heat development of the photothermographic element.
U.S. Pat. No. 3,531,286 discloses the use of photographic phenolic or active methylene colour couplers in photothermographic emulsions containing p-phenylenediamine developing agents to produce dye images.
British Pat. No. 2,100,458 discloses the use of sulphonamidophenol and sulphonamidonaphthol dye-releasing redox compounds which release a diffusible dye on heat development. Various other dye-releasing systems have been disclosed e.g. U.S. Pat. Nos. 4,060,420, 4,731,321, 4,088,469, 4,511,650 and 4,499,180, often involving thermal generation of a basic substance.
British Pat. No. 2,100,016 discloses the use of dye-releasing couplers which, in combination with a reducing agent, release a diffusible dye on heat development.
It is an object of the present invention to provide alternative heat developable colour photographic materials capable of providing clear, stable colour images.